The scanning force microscope is one example in a broad category of scanning microprobes. Types of microprobes described to date are sensitive, inter alia, to magnetic, electrical, mechanical, geometrical, thermal, electrostatic and optical properties of the sample. In general terms, a scanning microprobe is an instrument that maps a spatially varying surface property into an image. In the scanning force microscope, as the sample moves in the horizontal plane relative to the probe, the probe which is mounted on a flexible cantilever is deflected due to the forces between the probe and the sample's surface. For the case of topographic imaging in the repulsive mode, the tip of the cantilever is scanned across the surface of the sample, the cantilever's deflection increases for peaks and decreases for valleys. The deflection of the cantilever is monitored with an optical deflection detection system. In this scheme, a laser beam from a laser diode is reflected off the top of the cantilever onto a position sensitive photodiode. A given cantilever deflection will correspond to a specific position of the laser beam on the photodiode. A servo loop, using the position detected, sends a correction signal to a transducer element which is connected to the sample holder to adjust the spacing between the sample and the probe in order to maintain constant force between probe and sample. This correction signal is recorded as the Z height of each point on the sample.
Most scanning force microscopes are connected to computer systems that create gray-scale images to represent the height information of a sample's surface. In a gray-scale image, x and y data form the horizontal plane and z data is displayed in a linear scale in which the brightness of the data point is directly correlated with the height of the surface structure. A darker data point corresponds to a lower height value, while a brighter data point corresponds to a higher height value.
Another possible computer-based topographic representation is three-dimensional image rendering of surfaces. In this representation, height is shown by superimposing the gray scale on a third (perpendicular) axis and rotating the display to an informative viewing angle. One can add a computer-generated artificial light source to cast shadows that enhance the three-dimensional rendition.
An optical deflection detection system used in the prior art includes a laser positioned directly above the cantilever, so that the laser's light directly impinges the cantilever. This is described in U.S. Pat. No. 4,935,634. The cantilever is positioned at an angle relative to the horizontal plane so that the path of the beam reflected off the cantilever is at an angle different than the incident beam. The reflected beam is then reflected off a fixed mirror so that the light strikes a detection device.
There are several disadvantages to the configuration of the prior art. Because the laser is positioned directly above the cantilever, the line of sight to the probe and the sample is obstructed so that any visual observation of the sample position relative to the probe can only be performed at an oblique angle or by disassembling the apparatus. Adjustment of the probe position can therefore be a very time consuming and awkward process. Furthermore, the adjustment accuracy is degraded by the off-axis view, particularly since the depth of field limits viewing to a narrow region.
Generally, the optical deflection detection device requires very precise positioning because the spot on the cantilever that must be hit by the laser beam is typically only 10 microns in diameter and the size of the laser diode beam is about 7 microns. Therefore, fine control of the angular positioning of the beam with respect to the cantilever and the photodetector is desirable. The mirror of the prior art, however, is fixed and therefore the detection system is difficult to adjust. Adjustment is by lateral positioning of the laser beam which may compromise mechanical stability.
Another disadvantage of the prior art is the complexity and inaccuracy of arrangements to adjust the position of the probe relative to the sample. Because of the strict requirements for mechanical stability, it is typical to employ a kinematic mount to couple the scanner and sample assembly to the probe and sensor head. Such arrangements require separate translation arrangements to permit probe positioning on the sample and typically these reduce the mechanical rigidity of the apparatus.